1. Field of the Invention
The present invention relates to the field of still video imaging, and especially to the timing and control circuits of an image sensor for a still video camera of the type that produces a movie image in addition to a still image.
2. Description Relative to the Prior Art
A still video camera, by virtue of its inherent video processing capability, is well suited for combination with an electronic viewfinder (see, for example, the camera disclosed in U.S. Pat. No. 4,456,931). This combination is particularly advantageous for previewing the picture as it will actually appear subsequent to video processing. Such a camera operates in two modes: a movie mode for producing a moving video image in the viewfinder and a still mode for producing and recording a still image. The movie mode is comparable to the operation of any motion picture video camera. In such a camera, the photosensitive region of an image sensor is continuously irradiated by scene light. Since the exposure time is fixed to accord with the video image rate (e.g., 1/30th second), the image sensor is repetitively processed at a corresponding image frequency (e.g., 30 frames/second).
In the still mode, it is desirable that the exposure time is controlled so as to vary the length of time the image sensor is exposed to light. The image sensor must therefore be processed, not repetitively, but only once in a manner that defines the required exposure time and produces a video signal corresponding to the exposed image. Since a common image sensor is preferably the signal source for both modes, one mode excludes the other, that is, the movie mode can only precede, or follow, the still mode. This fact of operation leads to the conventional timing sequence in a still video camera. For instance, in U.S. Pat. No. 4,456,931, a control circuit is disclosed which either provides one set of drive signals to the image sensor when the monitor (viewfinder) displays the image focused on the image sensor or another set of drive signals to initiate and process the still exposure. Though not showing an electronic viewfinder like that shown in the preceding patent, U.S. Pat. No. 4,541,016 discloses a full-frame image pickup device that is synchronized, in its still operation, to a continuously running vertical drive signal. Since the charge storage period of each field is treated independently of the other, this disclosure suggests that a solid state image pickup unit equipped for still image pickup could also provide moving image pickup like an ordinary video camera.
Despite such suggestions from the prior art, it is not a trivial matter to incorporate movie timing with still timing. Substituting one set of drive signals . . . the still drive signals . . . for another set of drive signals . . . the movie drive signals . . . is not a totally asynchronous operation, as might be implied in U.S. Pat. No. 4,456,931. If for no other reason than cost, duplication of timing circuits is preferably avoided. More importantly, the two sets of drive signals must be tied together for the system to make a smooth transition from movie to still and back. On the other hand, if the still drive signals are totally synchronous with the vertical drive signal, as is the case in U.S. Pat. No. 4,541,016, the still exposure must occur in lockstep with the moving image pickup function. This is unfortunate in that the desired beginning of a still exposure seldom coincides with the beginning of the vertical drive interval. Some delay, i.e., up to 1/60 second, could thus be encountered before actually beginning the still exposure.